1. Field of the Invention
The present invention relates to a switching mode power supply and more specifically relates to the pulse width modulation (PWM) of the switching mode power supply.
2. Background of the Invention
The PWM is a traditional technology used in switching power supplies for regulating outputs. Due to the restriction of environmental regulations, the power supply design for computers and other electrical products are required in order to equip the power management function to save energy. One of the major concerns of the power management is to save power in light load and no load conditions.
FIG. 1 shows a general switching power supply, in which a PWM controller 100 is used to control and regulate the output of the power supply. When the line voltage VIN is applied to the power supply, a capacitor 220 connected to a supply voltage pin VCC of the PWM controller 100 will be charged via a resistor 210. Once the supply voltage VCC in the VCC pin of the PWM controller reaches a start-threshold voltage, the PWM controller 100 will be turned-on and start to operate. After that, the auxiliary bias winding of the transformer 400 will supply the power for the PWM controller 100 through a rectifier 230. If the supply voltage VCC is lower than a stop-threshold voltage due to insufficient power from the auxiliary bias winding, the PWM controller 100 could be turned-off. A resistor 240connected in series with a switching transistor 300 is used to convert the switching current of the transformer 400 into a sense voltage VS, which is compared with a feedback signal VFB to achieve the current mode PWM control. The feedback voltage VFB is derived from the output of an optical-coupler 250. The input of the optical coupler 250 is connected to the output of the power supply VO through a resistor 290 and a zener diode 280 to form the feedback loop. The feedback voltage VFB controls the on-time (TON) of the PWM signal and regulates the output of the power supply.
The power consumption is a major concern for switching mode power supplies. Various losses such as the transformer core loss, the transistor and the rectifier switching losses, and the snubber loss, are directly proportional to the switching frequency F. The switching period T is the reciprocal of the switching frequency F. Increasing the switching period will reduce the power loss, however a maximum on-time (TON(max)) of the switching signal is required to be restricted to prevent saturating magnetic components such as inductors and transformers.
In order to increase the regulator efficiency, some methods such as varying the switching frequency and entering the xe2x80x9cpulse-skippingxe2x80x9d mode according to load conditions have been disclosed. For example, U.S. Pat. No. 6,100,675, xe2x80x9cSWITCHING REGULATOR CAPABLE OF INCREASING REGULATOR EFFICIENCY UNDER LIGHT LOADxe2x80x9d disclosed an oscillation frequency control circuit capable of varying an oscillation frequency of the oscillator circuit in response to load conditions. Another method is disclosed in U.S. Pat. No. 6,366,070 B1, xe2x80x9cSWITCHING VOLTAGE REGULATOR WITH DUAL MODULATION CONTROL SCHEMExe2x80x9d, which disclosed the regulator employs three operation modes which operate at constant switching frequency for heavy load conditions, use dual modulation control scheme for moderate load conditions and enter xe2x80x9cpulse-skippingxe2x80x9d for light load conditions. The disadvantage of foregoing prior arts are: (1) Varying the switching frequency without the limitation of maximum on-time may result in saturation of magnetic components and cause over-stress damage to switching devices such as transistors and rectifiers. (2) The modulation of switching frequency is only controlled by load conditions and is not correlated with the supply voltage. As the switching frequency is reduced too low for saving more power in light load and no load conditions, the auxiliary bias winding of the transformer or inductor might be unable to provide sufficient power for the PWM controller. Thus, to correlate the frequency modulation with both load conditions and the supply voltage is absolutely needed. (3) In light load and no load conditions, the switching frequency might decrease and fall into the audio band. If the magnetic components are not well impregnated, the audio band switching frequency might generate acoustic noises.
To prevent above shortcomings of prior arts, there exists a need for a better apparatus with less acoustic noises to Improve the efficiency and save the power consumption in light load and no load conditions.
According to the present Invention, a current-driven PWM controller is implemented by incorporating the PWM function with the power saving means in which the switching frequency is decreased In response to the decrease of the load. Furthermore, a current-driven technique is mainly used to minimize the circuitry and reduce the cost of the PWM controller. Operating most of the control signals in current mode greatly reduces the die size of the Integrated PWM controller circuit. An off-time modulator is provided for power saving which results in keeping constant the maximum on-time of the PWM signal and increasing the off-time of the PWM signal. Thus, the switching period in light load conditions is extended. The off-time modulation is designed as the function of a feedback current that is derived from the feedback loop and represents the load condition.
Accordingly, the off-time modulator comprises: a reference voltage associated with a first resistor which generates a first constant current and a second constant current. The first constant current subtracts a current mirrored from the feedback current and generates a first differential current. A first output current mirrored from the first differential current produces a feedback voltage through a second resistor. The feedback voltage is further used for the PWM control and generates the PWM signal. The second constant current subtracts another current mirrored from the feedback current and generates a second differential current for the purpose of the off-time modulation. A second output current mirrored from the second differential current is clamped below a maximum value that controls a minimum off-time for high load conditions. A minimum discharge current is further mirrored from the reference current. The minimum discharge current determines a maximum off-time for the switching signal. Additionally, the minimum value of the minimum discharge current is limited to prevent the switching frequency from falling into the audio band.
The input of a control circuit is connected to the supply voltage. The output of the control circuit is used to turn on/off the minimum discharge current in response to the state of the supply voltage. The minimum discharge current is disabled when the supply voltage is high. The minimum discharge current and the switching of the PWM signal are enabled once the supply voltage is lower than the threshold voltage. Therefore, an Insufficient power supplied from the auxiliary bias winding is avoided for the PWM controller.
Advantageously, the current-driven off-time modulation improves the efficiency and saves the power consumption of the power supply in light load and no load conditions. Meanwhile, the acoustic noise is reduced. The magnetic devices are prevented from saturation. Furthermore, due to the current-driven design, the complexity and cost of the controller circuit are both reduced.
It is to be understood that both the foregoing general description and the following detail description are exemplary and are intended to provide further explanation of the invention as claimed.